The present invention relates to integrated circuits (ICs) and, more particularly, to a solution that prevents electrical charge that collects on long signal lines that are interrupted by buffers, or repeaters, in an IC from damaging the transistors of the buffers.
In ICs, very long conductive signal lines are sometimes needed to connect certain drivers to certain receivers in the IC. Signals propagating along these long lines are delayed due to the combination of the capacitance and resistance of the lines. It is known to insert buffers, or repeaters, into the lines to reduce the signal delays. A buffer is a device on the IC comprised of field effect transistors (FETs) that are configured to drive the signal received by the buffer to reduce the delay. Generally, the delay increases in a non-linear fashion as the length of the line increases (i.e., the increase in delay is proportional to the length of the line squared). Therefore, the inclusion of buffers along long lines is relatively common in ICs manufactured using current IC manufacturing processes.
One of the problems associated with the use of buffers is that, when the lines are being put down during fabrication of the IC, electrical charge builds up on the lines. The process of putting lines down includes spraying charged metal ions onto the IC wafer. These charged ions collect on the gates of the transistors of the buffers and can punch holes in the gate oxide of the FETs, which damages the repeaters. One known solution to this problem is to fabricate diodes into the IC that are coupled to the lines at locations close to the buffers. The diode will pull enough of the charge off of the gate of the FET to prevent damage to the FET, and thus to the buffer.
Rule checker programs, which are used to check IC designs before the ICs are fabricated, are capable of determining whether a protection diode is needed in order to protect the buffer and, if so, the location at which the diode should be placed in the IC. One of the difficulties associated with using such tools to determine whether a diode is needed and, if so, where it is to be located, is that existing tools cannot always find a location for the protection diode. ICs typically have many blocks and layers, and there is not always a convenient location for the diode. Also, the diode should be close to the buffer, which also presents problems when trying to find a location for a diode.
It would be advantageous to provide a technique that ensures that a location for a protection diode always exists, that the location is close to the buffer and that eliminates the need to use rule checker tools to determine whether a protection diode is needed.
The present invention provides a method and apparatus for preventing buffers used to reduce delays on long lines of an IC from being damaged due to charge that collects on the buffers due to the capacitance and resistance of the lines. In accordance with the present invention, a protection diode is included directly in at least each buffer that used for this purpose.
Preferably, each buffer on the IC that functions to reduce long-line delays comprises a protection diode so that it is unnecessary to run a rules checker program or the like to determine whether a protection diode is need and, if so, where to place it. However, a rules checker program can be used to determine whether a buffer needs to be protected by a diode. Then, protection diodes could only be included in buffers that need them. Therefore, although the present invention obviates the need for using a rules checker program for this purpose, a rules checker program can be used with the present invention if desired. The present invention also obviates problems associated with trying to determine where to locate a protection diode, assuming that any location can be found for the diode.
These and other features and embodiments of the present invention will be described below with reference to the detailed description, drawings and claims.